利用富乙酸剩余污泥厌氧发酵液产中链脂肪酸

Conversion of acetate-rich waste activated sludge anaerobic fermentation liquor into medium-chain fatty acids

为实现剩余污泥的资源化利用,探索了混菌体系中以剩余污泥为底物连续产中链脂肪酸(MCFAs)的可行性。本研究基于乙醇/乙酸人工配制废水,采用热碱水解污泥-短期厌氧发酵-微生物碳链延长(CE)反应的"两相发酵"技术合成MCFAs,并逐步优化水力停留时间(HRT)与底物醇酸比以驯化厌氧污泥微生物。结果表明:在为期135 d的连续模式CE过程中,在醇酸比为2∶1的条件下,驯化期(PhaseⅠ~Ⅲ)的HRT由20 d逐步缩减至5 d后,典型CE微生物Clostridium sensu stricto;2成为优势菌种,其相对丰度升至65.21%,但己酸产率仅为775 mg·(L·d)^(-1);当醇酸比提高至3∶1 (PhaseⅣ),己酸产率升至1 402 mg·(L·d)^(-1),MCFAs产物选择性明显提高。将实验期(PhaseⅤ)系统中的底物置换为污泥厌氧发酵液,己酸产率依然稳定保持在1 400 mg·(L·d)^(-1),表明功能微生物组的结构稳定。宏基因组分析结果显示,逆向β-氧化(RBO)和脂肪酸生物合成(FAB)代谢通路均参与了CE过程的MCFAs合成;另外,相较于乙醇/乙酸人工配制废水,污泥发酵液可提高这2种代谢通路的关键酶丰度。本研究证实了污泥连续发酵产MCFAs的可行性,并阐明了过程中微生物的生态功能机制,可为污泥资源化利用提供参考。

To determine the feasibility of producing medium-chain fatty acids(MCFAs) from waste activated sludge in mixed culture, this study firstly optimized bioreactor parameters, i.e. hydraulic retention time(HRT)and ethanol : acid ratio, to assimilate anaerobes and later adopted the "two-stage fermentation " strategy, in which alkaline pretreated sludge was subjected to short-term acidogenesis and microbial chain elongation The results showed that during the 135-day chain elongation(CE) over the long term, with ethanol∶acid ratio=2∶1,when HRT was shortened from 20 d to 5 d(Phase Ⅰ~Ⅲ), the CE functional microbe, Clostridium sensu stricto;2, evolved as the dominant genus(relative abundance 65.21%). However, the maximal productivity of n-caproate was merely 775 mg·(L·d)^(-1). Subsequently, with ethanol∶acid ratio increased to 3∶1(Phase Ⅳ), the productivity of n-caproate boosted to 1 402 mg·(L·d)^(-1), demonstrating an increased product selectivity towards MCFAs. In PhaseⅤ(test phase), during which the substrate swamped from synthetic ethanol/acetate wastewater to sludge fermentation liquor(SFL), the n-caproate productivity maintained at 1 400 mg·(L·d)^(-1). Based on the metagenomics analysis, both reverse β-oxidation(RBO) and fatty acid biosynthesis(FAB) pathways were involved in microbial chain elongation for MCFAs production. Moreover, as compared to synthetic ethanol/acetate wastewater, SFL increased the relative abundance of some key functional enzymes for the RBO and FAB pathways. The present study provided the practical evidence for continuous production of MCFAs from waste activated sludge, and more importantly, it elucidated the microbial and ecological mechanisms.Taken together, it shed a light on the sludge-derived value-added chemicals for its valorization.